Trapping ion mobility spectrometer with parallel accumulation

ABSTRACT

The invention relates to the operation of trapping ion mobility spectrometers based on pushing ions by a gas flow against a counter-acting electric DC field barrier, preferably in combination with a mass analyzer as ion detector. The invention provides an additional RF ion trap upstream of the trapping ion mobility spectrometer, wherein the RF ion trap is operated as an accumulation unit in parallel with the trapping ion mobility spectrometer such that a first group of ions can be analyzed in the trapping ion mobility spectrometer while a second group of ions from an ion source are simultaneously collected in accumulation unit.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to devices and methods for the acquisition of massspectra of ions separated by their mobility.

Description of the Related Art

In document U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008), a relativelysmall ion mobility spectrometer is presented which is termed “trappedion mobility spectrometer” (TIMS). The length of the essential mobilityseparation unit, the separator tunnel, amounts to about five centimetersonly, not counting additional entrance and exit funnels. The separationof ions according to their mobilities is based upon a gas flow in thecylindrical separator tunnel which drives the ions from an ion source inan accumulation phase against a counter-acting electric DC field barrierwhile the ions are radially confined by a quadrupolar RF field. Aftershutting down the delivery of further ions, a scan phase starts, inwhich the electric DC field barrier is steadily decreased. Ions aredriven in the scan phase by the gas flow over the decreasing electric DCfield barrier, thereby releasing, successively, ions from low mobilitiesto higher and higher mobilities from being trapped by the barrier. Theions can be detected in an ion detector, resulting in a mobilityspectrum.

FIG. 1 schematically shows a preferred design and operation of atrapping ion mobility spectrometer as described in U.S. Pat. No.7,838,826 B1. The tube-like separator tunnel (11) between entrancefunnel (10) and exit funnel (12) amount to only 48 millimeters inlength; the inner diameter amounts to eight millimeters. The ionmobility separator tunnel (11) consists of a series of segmenteddiaphragms with quadrant electrodes (1), (2), (3), and (4), forgenerating a quadrupolar RF field. Ions (6) from a source (not shown)are introduced by capillary (8) together with a gas stream (7) into afirst vacuum chamber. A repeller plate (9) directs the ions into thefunnel (10); the gas flow (14) drives the ions into the separator tunnel(11). In the bottom part of FIG. 1, electric field profiles E(z) alongthe z-axis are shown for three phases of operation: In the accumulationphase (A) ions are blown by the gas flow (16) against the rising edge ofthe electric field profile between z locations (20) and (23). In a trapphase (B) of only one to two milliseconds, the inflow of ions is stoppedand ions assume their equilibrium position on the rising edge accordingto their mobility. The steadily decreasing profile voltage in the scanphase (C) releases ions in the order of increasing ion mobility over theplateau of the electric field between locations (23) and (24) andthrough the exit funnel towards an ion detector. Particularly, the ionsmay be measured by a mass spectrometer, e.g. a time-of-flight massspectrometer, resulting in a two-dimensional mass-mobility spectrum.Unlike many other trials to build small ion mobility spectrometers, thedevice by M. A. Park has already achieved ion mobility resolutions up toR_(mob)=250, a very high ion mobility resolution never achieved byconventional mobility spectrometers.

There is still a need for devices and methods operating with highestutility rates (duty cycle) of the ions generated in an ion source of amass spectrometer, thereby reducing the restriction of the mobilityresolution, in particular with an electrospray ion source coupled toliquid chromatography for analyzing complex samples in the field ofbottom-up proteomics.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a method for operating atrapping ion mobility spectrometer, comprising the steps: (a)accumulating ions from an ion source in an RF ion trap; (b) transferringat least a subset of the accumulated ions into a trapping ion mobilityseparator, in which the transferred ions are radially confined by an RFfield and are pushed by a gas flow against a rising edge of a firstaxial electric DC field barrier such that the transferred ions arespatially separated along the rising edge according to ion mobility; and(c) acquiring an ion mobility spectrum of the transferred ions bydecreasing the height of the electric DC field barrier while ions fromthe ion source are further accumulated in the RF ion trap.

The utilization rate of ions produced in an ion source, accumulated andsubsequently separated in an trapping ion mobility spectrometer knownfrom the prior art is limited by the ratio q=t_(a)/(t_(a)+t_(s)) whereint_(a) is the ion accumulation time in the trapping ion mobilityseparator and T_(s) is the scan time of the electric DC field barrier,during which ions cannot be accumulated in the trapping ion mobilityseparator. The utilization rate is also termed “duty cycle” here.According to the present invention, the duty cycle can be close to 100percent when no ions get lost in the RF ion trap (accumulation unit) andin the trapping ion mobility separator (scan unit).

In one embodiment, the ions are radially confined in the trapping ionmobility separator by a quadrupolar RF field. The ions can beaccumulated in a RF funnel located between the ion source and thetrapping ion mobility separator wherein the field at the exit of the RFfunnel is preferably adjusted to the quadrupolar RF field at theentrance of the trapping ion mobility separator. The ions are preferablyaccumulated in a linear RF trap having a quadrupolar RF field forradially confining the ions. RF fields with higher multipoles, like ahexapolar or octopolar RF fields, can further enhance the ion storagecapacity of the linear RF ion trap, but it is more elaborate to couplethem with the preferred quadrupolar RF field of the trapping ionmobility separator.

In another embodiment, steps (b) and (c) are repeated to acquire aseries of ion mobility spectra, in particular a time series of ionmobility spectra. The ion density can be determined as a function of ionmobility from one or more preceding ion mobility spectra and then usedto adjust the electric field profile of the rising edge of the firstelectric DC field barrier in order to minimize ion losses in thetrapping ion mobility separator during steps (b) and (c). The electricfield profile is preferably adjusted such that ions are decompressedwhere a high ion density is determined or where ions of interest aredetermined.

The ions can be axially trapped at the exit of the RF ion trap byapplying a repelling DC potential to an exit electrode of the RF iontrap during steps (a) and (c). Preferably, the accumulated ions arepushed by the gas flow against a rising edge of a second axial electricDC field barrier located in the RF ion trap. During steps (a) and (c),the height of the second DC field barrier is adjusted such that ions aretrapped along the rising edge of the second DC field barrier andspatially separated according to ion mobility. In step (b), at least asubset of the accumulated ions is transferred into the trapping ionmobility separator by reversing, turning-off or decreasing the height ofthe second electric field barrier. The electric field profile of therising edge of the second electric DC field barrier can also be adjustedto minimize ion losses in the RF ion trap during steps (a) and (c), inparticular after determining the ion density as a function of ionmobility from one or more preceding ion mobility spectra.

In another embodiment, the transferred ions are further analyzed in amass analyzer being located downstream of the trapping ion mobilityseparator. The transferred ions are preferably fragmented in afragmentation cell located downstream of the ion trapping mobilityseparator wherein the fragments ions are analyzed in a mass analyzerlocated downstream of the fragmentation cell. Precursor ions can beselected in an additional mass analyzer located between the trapping ionmobility separator and the fragmentation cell and then fragmented in thefragmentation cell.

In a second aspect, the invention provides a system comprising an ionsource, an RF ion trap and a trapping ion mobility separator. Thetrapping ion mobility separator comprises an RF field for radiallyconfining ions, an axially acting electric DC field barrier having arising edge with an increasing electric DC field and a gas flowcounter-acting the electric DC field at the rising edge. The RF ion trap(accumulation unit) is located between the ion source and the trappingion mobility separator and has a first mode of operation foraccumulating ions from the ion source and a second mode of operation fortransferring ions towards the trapping ion mobility separator.

In one embodiment, the RF ion trap (accumulation unit) is an RF funnelor a linear RF ion trap. The linear RF ion trap is preferably alignedalong a common axis with the ion trapping mobility separator whereinboth, the linear RF ion trap and the trapping ion mobility separator,comprise a tube for radially confining the gas flow along the commonaxis. The linear ion trap can be designed as an upstream extension ofthe trapping ion mobility separator having a length of more than threecentimeters, more preferably of more than five centimeters, up to tencentimeters. The linear RF ion trap can be composed of segmenteddiaphragms with quadrant electrodes as shown in FIG. 1. The innerdiameter of the linear ion trap is preferably adjusted to the innerdiameter of the trapping ion mobility separator. Preferably, theradially confining RF fields of the linear RF ion trap and the trappingion mobility separator are substantially quadrupolar.

The linear RF ion trap can comprise an electric DC field barrier havinga rising edge with an increasing axial electric DC field. The slope(axial gradient) of the electric field strength along the rising edge ofat least one of the electric DC field barriers is preferably notconstant in a substantial portion of the rising edge.

In another embodiment, the system can further comprise at least one massanalyzer located downstream of the trapping ion mobility separator. Themass analyzer can be one of an orthogonal time-of-flight mass analyzer,a quadrupole filter, an RF ion trap, an electrostatic ion trap and anion cyclotron resonance mass spectrometer and it is located downstreamof the ion mobility analyzer. The system can further comprise afragmentation cell located between the trapping ion mobility analyzerand a mass analyzer. The fragmentation cell can be configured tofragment ions by one of collisional induced dissociation (CID), photoninduced dissociation (PID), electron capture dissociation (ECD) andelectron transfer dissociation (ETD). An additional mass analyzer can belocated between the trapping ion mobility separator and thefragmentation cell for selecting precursor ions. The ion source of thesystem can, for example, be one of an electrospray ion source (ESI), achemical ionization ion source (CI), a matrix-assisted laserdesorption/ionization ion source (MALDI) and an electron impact ionsource (EI).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the design and operation of an ionmobility spectrometer according to prior art, as described in U.S. Pat.No. 7,838,826 B1 (M. A. Park, 2008).

FIG. 2 shows an embodiment of a mobility spectrometer according to thepresent invention, with an elongated tunnel (11), divided into anaccumulation unit (11 a) and a scan unit (11 b), and two voltage supplyunits (not shown) for the two tunnel units (11 a, 11 b), contacting thediaphragms at locations (31) and (34). Chains of resistors between thediaphragms in both tunnel units produce two axial electric DC fieldprofiles, shown in the bottom part of the figure. The operationcomprises two phases: In the accumulation and scan phase (D), ions froman ion source (not shown) are accumulated on the rising edge of theelectric field profile in the accumulation unit (11 a) while, at thesame time, ions in the scan unit (11 b) are scanned by decreasing thevoltage supplied to location (34) of the scan unit (11 b), therebyreleasing ions with higher and higher mobilities through the exit funnel(13) towards the ion detector. In the transfer phase (E), first thevoltage of the scan unit (11 b) is restored, and then the voltage of theaccumulation unit (11 a) is switched off to let the ions be driven bythe gas flow onto the rising edge of the electric field profile of thescan unit (11 b). The transfer is completed after only one millisecond,and the accumulation and scan phase may start again by switching on thevoltage at location (31).

FIG. 3 shows the dependence of the mobility resolution R for the deviceshown in FIG. 1, given for ions of low mobility (Ko≈0.5 cm²/Vs), on thescan time t_(s). The scan time t_(s) comprises the full scan from lowmobility (Ko≈0.5 cm²/Vs) to high mobility (Ko≈1.0 cm²/Vs). Usually, thegeneral interest is on ions of low mobility because they show thehighest variety of folding states. A scan time of 60 milliseconds overthe full range of mobilities achieves a mobility resolution of R≈80; amobility resolution of R≈125 needs a scan time of 360 milliseconds.

FIG. 4 shows two different profiles of the electric field, generatingdifferent ion densities near the top of the electric field profile. Inthe upper part (D), repeated from FIG. 2, the electric field strengthincreases linearly along the z-axis of the device, showing equal iondensity along the ramp, in case a mixture of ions with uniform mobilitydistribution is provided. In the bottom profile (F), the electric fieldincreases proportionally to z^(2/3). As indicated by the dots, high massions near the top of the ramp are uncompressed by the field proportionalto z^(2/3), whereas low mass ions are compressed at the foot of theramp. Near the top of the ramp, the space charge is diminished, andlosses of high mass ions are considerably reduced.

FIG. 5 again shows two different profiles of the electric field. In theupper part (D), repeated for reasons of comparison from FIG. 2, theelectric field strength increases linearly along the axis z of thedevice. Below, in part (F), the ramp of the electric field shows, in theaccumulation unit as well as in the scan unit, three linear fieldgradients. In the accumulation profile, the linear field gradients arelocated between z locations (50) and (51), (51) and (52), and (52) and(53) respectively, the lowest gradient near the top. In the scanprofile, the linear field gradients are positioned between z locations(55) and (56), (56) and (57), and (57) and (58) respectively. Near bothtops, ion density and space charge are diminished, compared with theprofile in part (D) at the top. In the second field profile, a furtherdecompression of ions is achieved by a shorter length L_(p) of theplateau.

DETAILED DESCRIPTION

The present invention provides an ion mobility spectrometer of the typedescribed in document U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008),additionally equipped with an upfront RF ion trap operated as anaccumulation unit. The accumulation unit operates in parallel with thetrapping ion mobility separator, preferably implemented as a separatortunnel. That is, while the trapping ion mobility separator is being usedto analyze a first group of ions according to ion mobility, theaccumulation unit is simultaneously collecting a second group of ionsfrom an ion source. This second group is then rapidly transferred—inabout a millisecond to the trapping ion mobility separator once theanalysis of the first group is complete. This allows the accumulationunit to collect ions nearly continuously while the trapping ion mobilityseparator analyzes ions nearly continuously. In a first preferredembodiment, the accumulation unit is located between an entrance funneland a separation tunnel. In a second embodiment, the entrance funnelitself can be designed to serve as the accumulation unit. In a thirdembodiment, the accumulation unit is located upstream of the entrancefunnel.

Particularly, in the case of the first embodiment with the accumulationunit located between the entrance funnel and the trapping ion mobilityseparator, the accumulation unit is preferably designed identical to thetrapping ion mobility separator (scan unit), just doubling the scan unitwith segmented diaphragm electrodes for generating a quadrupole RFfield, and doubling the voltage supply units (with voltage dividers) forgenerating two independent axial DC electric field barriers. The risingedge of the electric field barrier may not increase as a single linearramp, but may show a flatter gradient near the top of the barrier, todecompress the density of ions of low mobility. This decompression maybe used in both axial DC electric field barriers, in the accumulationbarrier and in the scan barrier, to reduce losses of ions. Experienceshows that the transfer of ions by the gas flow from the accumulationunit to the scan unit of the device only needs a single millisecond. Thedelivery of further ions from the ion source need never be stopped. Ifthe accumulation time can be increased to about 300 milliseconds, also ascan time of 300 milliseconds can be used, resulting in a high mobilityresolution of R_(mob)≈120. If no ions get lost in the accumulation unitand scan unit, the utilization of ions amounts to 100 percent.

The accumulation unit must not necessarily be identical with the scanunit of the device. As an example, ions can be accumulated in anoctopole or hexapole RF field, instead of a quadrupolar one. Hexapoleand octopole RF fields can take up more ions. They can be made shorterthan the scan unit, because it is useless to collect more ions than canbe held in the scan unit of the device.

If the entrance funnel (10) of FIG. 1 is used as the accumulation unit,only small design changes need to be made to adapt its size and functionto take up sufficient ions for the subsequent mobility analysis. Anaccumulation unit located upstream of the entrance funnel can bedesigned as a common RF linear ion trap comprising a quadrupole,hexapole or octopole RF rod system.

FIG. 2 shows an embodiment with an accumulation unit (11 a) between anentrance funnel (10) and separator tunnel (11 b). The accumulation unit(11 a) is designed identical to the mobility separator tunnel (11 b)(scan unit), just doubling the separator tunnel (11 b) with itssegmented diaphragm electrodes for generating a quadrupolar RF field,and doubling the voltage supply units (with voltage dividers) forgenerating two independent axial DC electric field barriers in series.In the lower part of FIG. 2, the field profiles for the two operationphases are shown; the accumulation and scan phase (D), and the iontransfer phase (E).

In FIG. 2, both rising edges (ramps) of the field profile are linear,which can result in an unfavorable high density of ions with lowmobilities near the top of the profile leading to high losses of theseions due to Coulomb repulsion (“space charge effect”). With longaccumulation times, the high mass ions, usually having low mobilitiesbecause of their high cross sections, get lost first because they aremuch less strongly focused by the pseudo-potential within thequadrupolar RF field than low mass ions, showing high mobility due totheir lower cross section. The effective force corresponding to thepseudo-potential is proportional to z²/m, z being the number ofelementary charges of the ion, and m their mass. High mass ions are onlyweakly focused, and thus are more sensitive to space charge repulsion,driving the ions radially out of the device. With standard highperformance electrospray ion sources, severe losses of high mass ionsalready start with accumulation times above 40 milliseconds. If theaccumulation and scan times are restricted to only 40 milliseconds, themobility resolution is restricted to only R_(mob)≈65, because themobility resolution of a trapping ion mobility spectrometer depends onthe scan time t_(s). The scan time t_(s) is defined here as the timeneeded to scan over a common mobility range from low mobility (Ko≈0.5cm²/Vs) to high mobility (Ko≈1.0 cm²/Vs). The dependence of the mobilityresolution on the scan time is shown in FIG. 3. Since the mobilityresolution R_(mob) also depends on the mobility K itself, the dependenceis shown for ions with Ko≈0.5 cm²/Vs.

To overcome losses of high mass ions, the present invention furtherproposes to decrease the density for low mobility ions near the summit,and to accept a higher density of high mobility ions near the foot ofthe axial DC electric field barrier. This goal can be achieved by anon-constant gradient (slope) of the electric field E(z) at the risingedge of the axial DC electric field barrier. A preferred embodiment isgiven by the profile in part (F) of FIG. 4. Here, the electric fieldincreases non-linearly according to the function E(z)˜z^(p), with anexponent p=⅔. This form of field ramp decreases the field gradient nearthe summit, decompressing the high mass ion density, and increases thefield gradient near the foot, compressing the low mass ion density. Thevalue p=⅔ is only an example; in fact, p may assume any value smallerthan 1.0. A favorable range for the exponent p is 0.3≦p≦0.9. If themixture of ions contains many high mass ions, a value of p=½ may be morefavorable; in the extreme, even p=⅓ may serve the purpose best.

Another embodiment of the invention is shown in part (G) of FIG. 5,showing a piecewise linear increase with three different gradients ofthe electric field. The field gradient between z positions (52) and(53), and between (57) and (58) near the top are flattest, decompressingthe ions with low mobility and reducing the space charge repulsion. Ofcourse, more than just three gradients may be applied.

In another embodiment, the electric field profiles are adjusted withrespect to the ion density in the mobility spectrum determined in one ormore preceding measurements. As an example for a variable field profile,the gradients of the piecewise linear parts of the rising edge, as shownin part (G) of FIG. 5, can be varied by two voltage generatorsdelivering each three adjustable voltages V₅₁, V₅₂ and V₅₄, and V₅₆, V₅₇and V₅₉, which are applied to diaphragms at corresponding positions onthe z axis. By adjusting, for example, the voltages V₅₁ and V₅₂ relativeto voltage V₅₄, a variety of field profiles for the accumulation unitcan be generated. If a preceding measurement shows a high density of lowmobility ions, the field gradient between positions (52) and (53) can bemade as flat as necessary to avoid losses. The preceding measurement canalso be used to adjust the field profile of the scan unit. As a matterof course, more adjustable voltages may be used at more z positions.

With a more complex device, voltages at all or a large part of thediaphragms along the z-axis may be generated by a series ofdigital-to-analog converters (DAC). Then, any field profile can begenerated, enabling an operation which reacts exactly to any densitydistribution of ions on the mobility scale.

For reason of comparison, the upper parts (D) of FIGS. 4 and 5 show afield ramp of linear increase, where it is to be seen that the ions of amixture having a uniform mobility distribution are uniformly distributedalong the rising edge. In practice, however, the mobilities of ions arerarely uniformly distributed.

Experience shows that the transfer of ions by the gas flow from theaccumulation unit to the scan unit of the device only needs somemillisecond or less when the gas has a velocity of about 100 m/s at apressure of some millibar. The delivery of further ions from the ionsource need never be stopped. If the accumulation time can be increasedto about 300 milliseconds without major losses of ions, a scan time of300 milliseconds can be used, resulting in a high mobility resolution ofR_(mob)≈120. If no ions get lost in the accumulation unit and scan unitat all, then the duty cycle would be to 100 percent.

The accumulation unit of the device must not necessarily be identical indesign with the scan unit. As an example, the ions can be accumulated inan octopole or hexapole RF field, instead of a quadrupolar one. Hexapoleand octopole fields can take up more ions. Segmented diaphragms forgenerating hexapole or octopole RF fields may be designed similar to thesegmented diaphragms (1, 2, 3, 4) used for the scan unit (11 b), onlywith more radial electrode segments. Because it is useless to collectmore ions than can be held in the scan unit (11 b) with a quadrupolar RFfield, the multipole accumulation unit can be made shorter.

In case an entrance funnel is used as accumulation unit, only smalldesign changes need to be made to the setup shown in FIG. 1 in order toaccumulate a sufficient number of ions from the ion source. The funnel(10) can be made longer to increase its volume, and an additionalvoltage supply unit may deliver a switchable repelling DC potential(stopping voltage) to the last diaphragm of the funnel (10).

An accumulation unit upstream of the entrance funnel can be implementedas a common linear RF ion trap comprising a quadrupole, hexapole oroctopole RF rod system. Storage devices of this type are well-known tothe specialist in the field and need not be further described here.

It should be mentioned that the scan need not necessarily be performedby linearly decreasing the voltage for the field profile. In the U.S.Pat. No. 8,766,176 B2 (M. A. Park et al, 2011), different scan modes arepresented. In particular, a scan mode with a partial slow scan speed canincrease the mobility resolution for ions in a smaller range ofmobilities. A “zoom scan” consists of three phases: a first partial scanwith highest scan speed, a second “zoom” phase with a reduced scan speedfor highest resolution, and a third phase with fast scan speed to emptythe trap.

It goes without saying that the capacity of the accumulation and scanunits also can be increased by enlarging the inner diameter of thedevice. A larger ion mobility spectrometer with higher RF voltages maystill be acceptable for a mass spectrometer, but the gas flow,increasing with the fourth order of the inner diameter, needs muchbigger and more expensive vacuum pumps.

The invention claimed is:
 1. A method for operating a trapping ionmobility spectrometer, comprising the steps: (a) accumulating ions froman ion source in an RF ion trap; (b) transferring at least a subset ofthe accumulated ions into a trapping ion mobility separator, in whichthe transferred ions are radially confined by an RF field and pushed bya gas flow against a rising edge of an axial electric DC field barriersuch that the transferred ions are spatially separated along the risingedge according to ion mobility; (c) successively releasing thetransferred ions according to their ion mobility by decreasing theheight of the electric DC field barrier while ions from the ion sourceare further accumulated in the RF ion trap; and (d) restoring the heightof the electric DC field barrier which triggers a consecutive transferof the accumulated ions from the RF ion trap into the trapping ionmobility separator.
 2. The method according to claim 1, wherein the ionsare radially confined by a quadrupolar RF field in the trapping ionmobility separator.
 3. The method according to claim 2, wherein the ionsare accumulated in a linear RF trap.
 4. The method according to claim 3,wherein the accumulated ions are radially confined in the linear RF iontrap by one of quadrupolar, hexapolar and octopolar RF field.
 5. Themethod according to claim 2, wherein the ions are accumulated in a RFfunnel wherein the field at the exit of the RF funnel is adjusted to thequadrupolar RF field at the entrance of the trapping ion mobilityseparator.
 6. The method according to claim 1, wherein, during steps (a)and (c), ions are axially trapped at the exit of the RF ion trap byapplying a repelling DC potential to an exit electrode of the RF iontrap.
 7. The method according to claim 1, wherein steps (b) and (c) arerepeated to acquire a series of ion mobility spectra.
 8. The methodaccording to claim 7, wherein the ion density is determined as afunction of ion mobility from one or more preceding ion mobility spectraand the electric field profile of the rising edge of the first electricDC field barrier is adjusted to minimize ion losses in the trapping ionmobility separator during steps (b) and (c).
 9. A method for operating atrapping ion mobility spectrometer, comprising the steps: (a)accumulating ions from an ion source in an RF ion trap; (b) transferringat least a subset of the accumulated ions into a trapping ion mobilityseparator, in which the transferred ions are radially confined by an RFfield and pushed by a gas flow against a rising edge of a first axialelectric DC field barrier such that the transferred ions are spatiallyseparated along the rising edge according to ion mobility; and (c)successively releasing the transferred ions according to their ionmobility by decreasing the height of the first axial electric DC fieldbarrier while ions from the ion source are further accumulated in the RFion trap, wherein, during steps (a) and (c), ions in the RF ion trap arepushed by the gas flow in the axial direction against a rising edge of asecond axial electric DC field barrier wherein the height of the secondaxial electric DC field barrier is adjusted such that ions are trappedalong the rising edge of the second axial electric DC field barrier andspatially separated according to ion mobility.
 10. The method accordingto claim 9, wherein, in step (b), ions are transferred into the trappingion mobility separator by reversing, turning-off or decreasing theheight of the second axial electric DC field barrier.
 11. The methodaccording to claim 9, wherein the ion density is determined as afunction of ion mobility from one or more preceding ion mobility spectraand the electric field profile of the rising edge of the second axialelectric DC field barrier is adjusted to minimize ion losses in the RFion trap during steps (a) and (c).
 12. The method according to claim 9,wherein the transferred ions are further analyzed in a mass analyzerbeing located downstream of the trapping ion mobility separator.
 13. Themethod according to claim 9, wherein the transferred ions are fragmentedin a fragmentation cell located downstream of the ion trapping mobilityseparator and the fragments ions are analyzed in a mass analyzer.
 14. Asystem comprising: an ion source; an RF ion trap; and a trapping ionmobility separator comprising an RF field for radially confining ions,an axially acting electric DC field barrier having a rising edge with anincreasing electric DC field and a gas flow counteracting the electricDC field at the rising edge; wherein the RF ion trap is located betweenthe ion source and the trapping ion mobility separator and wherein theRF ion trap has a first mode of operation for accumulating ions from theion source and a second mode of operation for transferring ions towardsthe ion trapping ion mobility separator; and wherein the RF ion trapcomprises an axially acting adjustable electric DC field barrier havinga rising edge with an increasing electric DC field and a gas flowcounteracting the electric DC field at the rising edge.
 15. The systemaccording to claim 14, wherein the RF ion trap is an RF funnel.
 16. Thesystem according to claim 14, wherein the RF ion trap is a linear RF iontrap which is aligned along a common axis with the trapping ion mobilityseparator and wherein both, the linear RF ion trap and the trapping ionmobility separator comprise a tube for radially confining the gas flowalong the common axis.
 17. The system according to claim 14, wherein theslope (axial gradient) of the electric field strength along the risingedge of at least one electric DC field barrier is not constant at asubstantial portion of the rising edge.
 18. The system according toclaim 17, wherein the radially confining RF fields of the linear RF iontrap or the trapping ion mobility separator are substantiallyquadrupolar.
 19. The system according to claim 14, further comprising atleast one of a fragmentation cell and a mass analyzer located downstreamof the ion mobility separator.
 20. The method according to claim 1,wherein the transferred ions are further analyzed in a mass analyzerbeing located downstream of the trapping ion mobility separator.
 21. Themethod according to claim 1, wherein the transferred ions are fragmentedin a fragmentation cell located downstream of the ion trapping mobilityseparator and the fragments ions are analyzed in a mass analyzer.